The below two figures indicate known exoplanets which enter their
optimistic Habitable Zones. The
size of the points linearly increases with the percentage of time
spent within the Habitable Zone. The plots show the mass of the planet
(left) and orbital eccentricity (right) versus the orbital
period. Click on the images to view the animated versions, which show
how our knowledge of Habitable Zone planets has increased with
time.

The below two figures show the dependence of effective stellar flux
received by the planet on stellar mass (left) and planet radius
(right). The green points are those planets which spend more than 50%
of their orbital phase within
the optimistic Habitable Zone. Red
and blue points are planets interior and exterior to the Habitable
Zone respectively. The dashed crosshairs show the location of
Earth. Click the images to view the larger versions.

The below figures show a Hertzsprung-Russell (H-R) diagram (left)
and a histogram of the discovery rate (right). The green represents
planets which spend more than 50% of their orbital phase within
the optimistic Habitable Zone. Red
and blue represent planets interior and exterior to the Habitable Zone
respectively. The dashed crosshairs in the H-R diagram show the
location of the Sun. Click the images to view the larger versions.